The experiments of this proposal intend to define precisely where and how HIV persists in the context of long-term antiretroviral therapy (ART). Although the impact of HIV infection has been substantially reduced by the introduction of effective ART, the virus is not completely eliminated from the host. Rather, persistent virus remains harbored in a transcriptionally silent form within multiple subpopulations of lymphoid and myeloid cells that are continuously moving between the peripheral circulation and specific tissue sanctuaries. The rationale for our approach is prompted by the observation that the reservoir of persistent HIV seems to vary depending on both the time when ART is started relative to the time of infection, and on the duration of time that it is thereafter maintained. Gien existing data sets, we hypothesize that the constellation of cells harboring HIV is formed in part by physiologic mechanisms of central and peripheral cell homeostasis, regulated by a complex combination of homeostatic signals including those provided by interleukin 7 (IL-7) and 15 (IL-15), which act differently as a function of ART initiation and duration. If so, then targeted interventions to eradicate HIV will be informed by further definition of the interactions that occu between the virus and various subpopulations of T and myeloid cells found in vivo, and of the way in which these interactions may change as a function of time and duration of treatment. In the experiments of this proposal, we intend to study the mechanisms that modulate the homeostasis of cells harboring latent HIV genomes, and how the timing of initiation and duration of effective ART impact upon the propensity of HIV to enter and persist in some but not all subpopulations. Experiments associated with three Specific Aims will be pursued: (1) to evaluate how the time of initiation and the duration of effective ART affect the distribution and composition of the persistent HIV reservoir; (2) to evaluate the turnover and differentiation capacity of CD4+ T cells harboring persistent HIV in vivo; and (3) to investigate the role of the ?c cytokines, IL-7 and IL-15, on the establishment and maintenance of the persistent HIV reservoir. We anticipate that the data generated by this study will provide new insights into the mechanisms by which HIV persists, even in the setting of ART. Within a five-year period of time, we should be in a position to definitively state whether or not the timing of initiation and duratin of ART dictate the composition and turnover of the persistent HIV reservoir, thereby defining the landscape on which various strategies to eradicate HIV must operate.